Typical single domain LCDs include a front alignment film and a rear alignment film. For example, see U.S. Pat. Nos. 5,594,568 and 5,576,861, the disclosures of which are incorporated herein by reference. In single domain LCDs, the front alignment film typically has a buffing or orientation direction which is substantially perpendicular (i.e. approximately 90.degree. angled) to the buffing or orientation direction of the rear alignment film. In other words, the buffing directions of the front and rear alignment films are defined in parallel planes, but differ in direction by approximately 90.degree.. Unfortunately, such single domain displays suffer from non-symmetric viewing cones. Transmission changes with viewing angle, which creates regions of low contrast and/or regions of gray level reversal. This non-symmetry is caused in part by mono-domain or single domain alignment in typical 90.degree. twisted nematic LCDs, and the resulting liquid crystal (LC) director profile.
Mechanically buffed polyimide layers are typically used as alignment/orientation films. These alignment layers are coated on the interior of opposing substrates of the display, and are intended to be in immediate contact with the directly adjacent LC (i.e. liquid crystal material). Cigar-shaped molecules of the LC are adjacent to the alignment layers and take on the same orientation as the corresponding alignment layer adjacent thereto. An anchoring force of the polyimide holds the immediately adjacent LC molecules in a fixed position. LC molecules not in contact with the alignment layers are not anchored, and these molecules can change orientation with an applied voltage as is known in the art.
A significant disadvantage of mono-domain or single domain twisted nematic LCDs is a non-symmetric director profile in the vertical direction. Such non-symmetry creates an angular dependent phase retardation. Polarized light incident on such an LCD from different viewing angles will experience different phase retardation. The result is that light exiting the LCD at different angles will have different polarization states and thus cannot be equally transmitted to a viewer through the polarizer or analyzer on the viewer side of the display. Such asymmetric LC director profile translates into an asymmetric display transmission as a function of viewing angle. Contrast ratio losses for some viewing angles results. Image reversal at certain viewing angles can also occur. Both of these effects are detrimental to image quality and viewing characteristics.
It is known in the art that director profile symmetry can be improved by having multi-alignment directions within a single pixel (i.e. multi-domain or two-domain LCDs). For example, see U.S. Pat. Nos. 5,589,963; 5,309,264; 5,479,282; and 5,249,070, the disclosures of which are hereby entirely incorporated herein by reference. In multi-domain LCDs, although each individual alignment direction produces non-symmetry, the combined or overall optical effect of the multi-domains creates a substantially symmetric viewing cone having substantially uniform contrast and gray levels over viewing angles.
Unfortunately, prior art multi-domain systems having been difficult to commercially implement for at least the following reasons. Firstly, prior art mechanical rubbing used to create alignment directions is undesirable. It is very difficult to achieve more than one rubbing direction per pixel or subpixel using such mechanical rubbing. It has been suggested that in order to achieve multi-domain feature via mechanical rubbing, first a polyimide coated substrate must be rubbed in a first direction, then a mask must be aligned with pixel or subpixel features and another step of rubbing performed in a second direction which is perpendicular to the first direction. Such mask use and alignment is difficult, inefficient, and burdensome. Additionally, the second mechanical rubbing can damage the alignment properties of the first rubbing. An additional shortcoming of mechanical rubbing is that each alignment direction requires a separate mechanical buffing step (i.e. at least two buffing steps are required), with this large number of steps increasing cost and decreasing yields of LCD manufacturing.
Photo-alignment methods can be utilized to create multi-domain displays. However, such methods are very difficult to implement in a manufacturing environment. For example, disadvantages of photo alignment methods include the fact that material for such methods is often unavailable, and that the requirement for expensive high irradiance polarized ultraviolet (UV) exposure systems is undesirable. Long exposure times also limit manufacturing throughput. Another shortcoming of photo alignment methods is pre-tilt degradation over time which is caused by exposure to the sun or other UV sources.
Prior art FIGS. 1(a) and 1(b) illustrate a two-domain LCD from U.S. Pat. No. 5,589,963. The display of FIGS. 1(a) and 1(b) includes rear polarizer 1, front polarizer or analyzer 3, liquid crystal layer 5, rear substrate 7, front substrate 9, liquid crystal molecules 11, rear rubbing direction 13, rear rubbing direction 15, front rubbing direction 17 which is perpendicular to rubbing direction 13, and front rubbing direction 19 which is perpendicular to opposing rubbing direction 15, and compensator 21. There are two pixels/subpixels 20 illustrated in FIG. 1(a), with each square pixel/subpixel 20 having two different rub/orientation directions on each side of the LC layer. For example, each pixel/subpixel 20 includes rub directions 13 and 15 on the rear side of the LC and corresponding rub directions 17 and 19 on the other side of the LC. Unfortunately, the two-domain LCD of FIGS. 1(a) and 1(b) suffers from the problems discussed above.
It is apparent from the above that there exists a need in the art for an improved multi-domain liquid crystal display (LCD), and an improved method of making same. Such an improved display, and corresponding method, would enable a multi-domain display to be made with only one buffing step required per substrate. Additionally, it would be useful to be able to vary or adjust buffing directions so as to be able to achieve a wide range of arbitrary alignment directions without having to mechanically buff or photo align orientation materials/layers. It would also be advantageous to have a photoimageable or photopatternable alignment layer on each substrate, so that the alignment layer, after having been oriented, could be patterned (e.g. via photo-imaging using photolithography) into a plurality of pixilated or discrete members to form an array.
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.